This disclosure relates to an image forming apparatus that forms an image on a sheet and to a light scanning device that scans an object to be scanned with light.
Image forming apparatuses, such as printers and copiers, often have an image bearing member that has an image to be transferred to a sheet, and an exposure unit that irradiates the image bearing member with light to form an electrostatic latent image thereon. High-speed image forming is achieved by making the exposure unit form an electrostatic latent image on the image bearing member at high speeds.
A typical exposure unit includes a housing and various optical elements accommodated in the housing. Such optical elements include, for example, a light-emitting element that emits light to form an electrostatic latent image and a rotating mirror that scans the image bearing member with light. Due to the emission of light from the light-emitting element and rotation of the rotating mirror, heat is generated and stays in the housing.
Because, as mentioned above, the exposure unit includes various optical elements, the housing of the exposure unit needs to be highly dustproof. Thus, the housing has a sealed structure. However, such a sealed structure encourages the heat to stay in the housing. Deformation of the housing due to the heat may change the optical settings of optical elements in the housing. Hence, it is preferable to use a resin having high heat resistance as the resin constituting the housing. In this case, the housing of the exposure unit may be relatively expensive. In particular, when the rotating mirror in the exposure unit is operated at high speed to increase the image-forming speed, the amount of heat accumulated in the housing may increase. Thus, the housing may be made from a resin having greater heat resistance. Consequently, an increase in the maximum image-forming speed achieved by an image forming apparatus (hereinbelow, “maximum image-forming speed”) may cause an increase in cost of the housing.
A reduction in size of the housing of the exposure unit may reduce the cost of the materials of the housing. However, such a reduction in size of the housing can further encourage the heat to stay in the housing.
Most optical elements in the exposure unit are made from an optical resin. Hence, the optical elements in the exposure unit may be made from a highly heat resistant optical resin, taking into consideration the heat accumulated in the housing. Therefore, an increase in image-forming speed may cause an increase in cost of the optical elements, as therefore the housing.
The relationship between the image-forming speed and the cost of material of the exposure unit, as described above, may be an obstacle to increasing the maximum image-forming speed.